The invention relates to solid state laser medium, particularly to gadolinium scandium gallium garnets, and analogs thereof, used in medium and high average power lasers with large apertures.
Gadolinium scandium gallium garnet, codoped with neodymium and chromium ions, is a leading candidate as a high average power laser medium because it has a high efficiency. The medium couples well to flashlamp pumps, since the chromium dopant absorbs broad energy bands and can transfer the energy to the neodymium dopant as the active lasing ion. Gadolinium scandium gallium garnet has the stoichiometric composition: Gd.sub.3 Sc.sub.2 Ga.sub.3 O.sub.12. However, in growing large cystals, or boules, problems with growth morphology have been encountered. Thus, spiraling of the boule appears when crystal growth exceeds about one to about two inches of boule diameter. This spiral morphology interferes with laser applications and makes most of the crystal useless.
Garnet materials doped with a rare earth, actinide or transition metal are known in computer applications for magnetic bubble memories and for tunable lasers. U.S. Pat. No. 4,490,822, issued Dec. 25, 1984, inventors Walling et al., discloses a wavelength-tunable laser where the laser medium is a single crystal of Y.sub.3 Ga.sub.5 O.sub.12 doped with Cr.sup.3 +. Drube et al., Optics Communications, 50, pp. 45-48 (1984) describes a tunable laser of Cr.sup.3 +doped Gd.sub.3 Sc.sub.2 Al.sub.3 O.sub.12. The just-mentioned two publications describe growth of the respective laser medium by the Czochralski technique. Honda et al., J. Appl. Phys., 51, pp. 896-897 (1980) describes neodymium doped Gd.sub.3 Ga.sub.5 O.sub.12 crystals also grown by the Czochralski method.
Where the laser medium is utilized in relatively small sizes, such as the 4 mm diameter by 55 mm long rod described by Honda et al. above and the 10 mm diameter by 50 mm long crystal described by Drube et al. above, then the spiral problem generally tends not to occur when a boule is grown. However, in applications where relatively large crystals are desired, then the Czochralski growth technique results in undesired spiral morphology during boule growth over about 2.5 cm boule diameter.